The use of a patterned sealing roll and an anvil roll that utilize pressure to effect seals along wrapped articles is well known in the product packaging arena. Patterned sealing rolls and anvil rolls utilize a combination of pressure and temperature to form the seals. Typically, the seal is accomplished by pressing a sealing roll and an anvil roll together with either a constant loading force or at a very small constant gap to create pressure on substrates or workpiece layers used in forming a wrapper and being fed between the sealing roll and the anvil roll. The high pressure on the substrates extrudes the substrates, creating a very thin membrane of substrate between the pattern surface of the sealing roll and the opposing sealing roll, which serves as an anvil. It is theorized that the rapid extrusion creates molecular friction and heat within the substrate membrane, causing the substrate to momentarily melt at the bond sites. The sealing roll and the anvil roll can also be heated to provide additional heat to what is typically a very thin membrane of the substrate. Since the membrane is very thin, there is adequate heat transfer to fuse and mix the substrate layers even though the residence time is very short. Higher process speed creates more internal friction, but less residence time. Typically with heated systems, a higher temperature setpoint is used for the rolls with higher speed. The pattern of the sealing roll has relief areas or grooves around the perimeter of the pattern to provide sufficient void volume for the extruded/melted material that is being extruded, melted and mixed, thereby forming a bead of molten material around each bond site. Once the bond site passes the rolls, the bead of molten material quickly cools and forms a bond of two or more layers of substrate.
While achieving a minimum seal strength is desirable to ensure adequate wrapper integrity and avoid premature opening of the product package, achieving too strong of a bond can be detrimental, as it can increase the difficulty of opening the wrappers, i.e., peel strength, to levels that are unacceptable to consumers. It is recognized that, during the service life of a given patterned sealing roll and anvil roll, the bond imparted to workpiece layers tends to increase peel strength. The problem of the peel strength increasing is believed and understood to be attributable, in part, to the increased area of the exposed textured surface of the patterned roll as the protuberances, or individual raised elements forming the textured patterns of the roll, wear down, as well as to increased contact due to pitting of the surface of the anvil roll, as explained in further detail below.
With respect to wrappers for products having two or more sealing zones of varying peel strength, the desirability of controlling the seal strength of one of the sealing zones may be more acute than in another of the sealing zones. For instance, it may be desirable for a first sealing zone of the wrapper to have a higher peel strength than an adjacent second sealing zone. The relatively low peel strength of the second sealing zone is conducive to initiating peeling of the wrapper to access a product contained therein, while the relatively stronger bond formed along the first sealing zone is preferably a more permanent seal.